Magnetism and spin-gap behaviour in the layered Pr T Al (T = Fe, Ru, Os) compounds with the distorted Kagomé lattice.

We have investigated the physical and magnetic properties of the distorted Kagomé lattice structure compounds; Pr[Formula: see text]Al (T  =  Fe, Ru, Os) which crystallize in the hexagonal GdRuAl-type structure with space group P6/mmc (No. 194). The three compounds show long-range magnetic orderings of FM type at T   =  36.5 K (Fe), 39 K (Ru) and 37 K (Os) as indicated by the temperature dependences of magnetic susceptibility, [Formula: see text], specific heat, C (T) and electrical resistivity, [Formula: see text]. PrRuAl shows an additional anomaly at 7 K which is associated to possible spin reorientation of some of the Pr moments. We also observed an additional anomaly in PrFeAl at 132 K which might be associated to the formation of a spin density wave. The Sommerfeld coefficient, [Formula: see text] extracted from C (T) in the temperature range immediately above T indicate heavy fermion behaviours in Pr[Formula: see text]Al (T  =  Fe, Ru, Os). [Formula: see text] of the three compounds show spin-gap behaviours below T of comparable energy gaps magnitude.